The first important change occurs when the aircraft reaches
a point within 30 NM of the destination airport. At this
point, regulations require that every GPS-based FMS/
RNAV unit increase its sensitivity and integrity monitoring
(receiver autonomous integrity monitoring, or RAIM, which
continuously checks GPS signal reliability and alerts you if
RAIM requirements are not met). If the system determines
that RAIM requirements are met, the FMS/GPS RNAV unit
automatically switches from en route sensitivity to terminal
sensitivity within 30 NM of the destination airport. Terminal
mode increases the sensitivity of the course deviation
indicator (CDI) from 5 NM to 1 NM. The FMS/GPS RNAV displays an annunciation to let you know that it has switched
from en route sensitivity to terminal mode.

Approach Mode

The second important change occurs 2 NM prior to reaching
the final approach waypoint. At this point, the FMS/GPS RNAV unit automatically switches to approach sensitivity.
At this stage, the FMS/GPS RNAV further increases RAIM
requirements, and increases the CDI sensitivity from 1 NM
to 0.3 NM (i.e., a full-scale CDI deflection occurs if you are
0.3 NM or more from the desired course).

As long as the annunciation for approach mode is displayed,
you may continue the approach. If, however, the computer
fails to switch to approach mode, or the approach mode
annunciation disappears, you must fly the published missed
approach procedure. You are not authorized to descend
further or to the MDA. Making changes to the FMS/
GPS RNAV after reaching the 2 NM point could result in
automatic cancellation of the approach mode.

Approach Not Active

If you arrive at the final approach waypoint and the approach
mode is not active, you must fly the missed approach
procedure. There should be no attempt to activate or
reactivate the approach after reaching the final approach fix
using any means—simply fly the missed approach procedure.

Vectored Approaches

As in conventional approaches, it is common for air traffic
control to issue vectors to a GPS or RNAV (GPS) final
approach course. Flying a vectored GPS or RNAV (GPS) approach is a simple matter of using the course intercept
technique described in the previous section. The technique
is illustrated again in Figure 3-51.

Many FMS/GPS RNAV units offer an automated solution to
the problem of flying an approach in which the pilot receives
vectors to the final approach course. Once ATC begins
providing vectors to intercept the final approach course, you
should activate the computer’s “vectors-to-final” feature,
which draws a course line along the final approach course.
This feature helps you maintain situational awareness while
being vectored because the assigned heading is clearly seen
in relation to the final approach course. As already noted,
you should monitor carefully to ensure that the FMS/GPS
RNAV unit switches to approach mode within 2 NM of the
final approach fix (FAF).

Figure 3-51 shows the vectors-to-final feature. The procedure
required to use the vectors-to-final feature is illustrated in
Figure 3-52.

When set to use the vectors-to-final feature, many FMS/
GPS RNAV units automatically set the FAF as the active
waypoint; and set the final approach course as the desired
track to the active waypoint.